In the detection of optical defects in glass articles, such as bottles or jars, it has been the practice to illuminate the jars, usually with a diffuse backlight, and view the container either optically or with a light sensitive pickup.
One such disclosure is U.S. Pat. No. 4,378,493 dated Mar. 29, 1983, where a system for illuminating the full height of a container placed in an inspection position is shown and described. The source disclosed consists of a plurality of incandescent bulbs behind a frosted glass plate, thus producing generally a relatively large diffuse source for backlighting the container in the inspection position. With this diffuse backlighting arrangement, the side of the container adjacent the light, which may have refractive defects in it, will not enter into or affect the light emanating from the forward or opposite wall of the container to any appreciable extent. A vertical, linear array camera, focused on the front wall of the container, will provide an image of the wall onto the vertical array of pixels in the camera. The pixels then are serially interrogated and adjacent pixels are compared with respect to their output, which is a function of the light received thereon, and in this manner, light which is reflected or sufficiently refracted by defects in the container wall in view of the camera, will be made apparent by the output of the linear array. Of course, this particular arrangement requires rotation of the container about its vertical axis in order to provide a circumferential scan of the entire container side wall and viewing area, which may also include the neck and shoulder area of the container. In this system, when a reflective defect, such as a check or an absorptive defect, such as a stone, appears in the wall of a container, as that portion of the wall is moved through the viewing area of the camera, the pixels upon which the wall is being focused will see areas of darkness caused by the reflection or absorption of the illuminated light out of the line of sight of the pickup. In this way, as previously stated, by comparing the output of adjacent pixels one can determine where the defect lies in a vertical plane and also to a great extent can determine the size of the defect as well. The pixels are scanned at a sufficient rate so that essentially every area of the bottle is viewed and most defects actually will span more than a single scan and will appear in several successive scans.
It should be remembered, however, that the light which reaches the forward wall of the container has come from a diffuse source of finite extent and therefore the intensity of the defect region is not materially affected by most light refracting effects in the object. This is particularly apparent when one considers that most optical inspection systems which are looking for dirt in the bottom of the container use a differ source positioned below the upright container so that lettering, such as factory and mold numbers, will not be visible from above the container where the optical transmission analyzer is located.
It has also been the practice to optically detect defects, such as checks, in various portions of glass articles by focusing a beam of light onto an area of the article at a particular angle and then positioning a pickup, such as a photocell, at approximately a 90.degree. angle with respect to the direction of the focused light. In this arrangement, such as is shown in U.S. Pat. No. 3,245,533, the light will be reflected from the defect onto the photocell, thus indicating the presence of a reflective defect. This has been the typical system for examining the finish and heel portions of glass containers in the past and the focused light will be reflected by a check into the photocell as the container is rotated about its vertical axis, in station, where the inspection setup is provided. It should be understood that the defects which are being detected are those typically termed checks caused usually by thermal shocks during the formation of the container generally by the touching of the hot glass after forming by a cold piece of handling equipment. Generally speaking, checks are reflective if their opposed surface separation is at least a half wave length of light. If the separation is less than a half wave length, the light would pass through and the defect would not reflect light and therefore not be detectable. Another defect which is picked up by the use of specular, focused light are those surface defects produced in glass containers which will cause the focused light to be refracted out of the direction in which it is being transmitted to the container and the placing of the pickups at positions such that refraction, for example, from a "line-over-finish" defect, such as illustrated in U.S. Pat. No. 3,302,787, will be detected.
In the inspection of flat glass articles such as television faceplates or architectural glass, it has been customary to illuminate the article with a beam of focused light and then sweep the focused light across the width of the article while moving the article at right angles to the scanning beam. In this way, nearly all of the glass surfaces will be covered. The light passing through the article is picked up by a complementary scanning photocell. Such a system is shown in U.S. Pat. No. 3,199,401 to Pittsburgh Plate Glass Co. It should be noted that the system requires that an angular illumination be used to avoid reflections that might give erroneous readings. The movement of slightly wavy appearing surfaces into the view of the light and pickup will cause refraction of the focused light and result in the pickup being without illumination during these periods. Whether these are commercially unacceptable becomes a matter of concern, and it would be advantageous to have an inspection system where the defects that are of the type which make the product unsatisfactory for its intended purpose, are enhanced and discriminated from those refractive effects that are not severe.
In order to provide special illumination techniques, one may employ a space invariant system as disclosed and claimed in copending application Ser. No. 634,930, filed July 24, 1984, the disclosure of which is incorporated herein by reference thereto.
In the above cited application, a diffuse source of uniform illumination intensity is provided with a hard edge mask over the source which limits the distribution of ray directions or angular spectrum of the plurality of collimated beams produced.